A standard procedure to manufacture highly esthetic dental restorations based on zirconia is milling pre-sintered zirconia mill blanks with an exactly defined shrinkage and fire that work piece to final density and wanted dimensions. The quality of the milled structure typically depends on the bending strength of the mill blank used. The bending strength of the mill blank can be determined e. g. using a punch-on-3-balls test according the ISO standard 6872.
For grinding with e. g. diamonded burrs the bending strength of the mill block is typically from 60 MPa up to final strength of zirconia with a preferred upper limit round about 110 MPa. Grinding can be performed e.g. with grinding machines like CEREC™ InLab (Sirona Corp.) and E4D (D4D Corp.).
Milling can typically be performed if the bending strength of the mill block is within a range of 15 to 50 MPa. Milling can be done using e.g. the Lava™ system of 3M ESPE.
In EP 1 206 223 A1 (3M ESPE) it is outlined that for milling the raw breaking resistance of the dental mill blanks is usually within a range of 15 MPa to 30 MPa. If milling is performed outside these ranges, the obtained structure is often not milled adequately and may show chipping.
In US 2004/0119180 A1 (3M ESPE) a process of producing a denture comprising the steps of providing a mill blank having a raw breaking resistance of 31 to 50 MPa, milling the blank and sintering the milled blank is described.
U.S. Pat. No. 5,702,650 (Hintersehr) a process for producing ceramic dental prostheses are described by shaping a porous unfinished piece made out of ZrO2, Y2O3, HfO2, dense sintering or infiltration of the unfinished piece and reworking the unfinished piece by means of a rotating tool is described.
In U.S. application 61/545,243 (3M IPC) aerogels, calcined and crystalline articles and methods of making the same are described. The content of this application is herewith incorporated by reference.
US 2011/0269618 (Knapp et al.) relates to nano-crystalline dental ceramics, where the nanocrystals are formed by vaporization.
However, there is still room for improvement especially with regard to the requirements to be fulfilled with respect to modern dental materials.